The emergence of antibiotic resistant bacteria is one of the most challenging public health problems affecting humankind in the 21st century. Among these bacteria, vancomycin-resistant enterococci (VRE) are one of the most difficult organisms to treat in hospitals across the US. Only two antimicrobial compounds are currently FDA-approved for the treatment of VRE infections;namely, linezolid and quinupristin-dalfopristin (Q/D). However, the use of these two agents against VRE has been hampered by suboptimal therapeutic outcomes in severe infections, frequent occurrence of side effects and the emergence and widespread dissemination of linezolid- and Q/D-resistant VRE isolates. Daptomycin (DAP) is a lipopeptide antibiotic whose mechanism of killing involves the interaction with the bacterial cell membrane (CM) in a calcium-dependent manner. DAP is the only bactericidal antibiotic currently available with activity against VRE. Although DAP does not have an FDA-approved indication for the treatment of VRE infections, clinicians are often pushed to use DAP due to the lack of better alternatives to treat patients infected with VRE who are often severely ill and with important compromise of the immune system. The off-label use of DAP during VRE therapy has led in several instances to the development of DAP resistance (DAP-R), thus, worsening the clinical scenario even further. Our long- term goal for this grant application is to understand the molecular events that lead to the development of DAP- R during VRE therapy to be able to i) design improved therapeutic strategies to prevent the emergence of DAP-R, and ii) identify new potential targets for antimicrobial development in the future with the aim of protecting the efficacy of DAP against VRE. Based on the information gathered from the comparative whole- genome, CM and cell envelope ultrastructural analysis of VRE clinical strain pairs of DAP-susceptible and DAP-resistant Enterococcus faecalis (VREfs) and E. faecium (VREfm), we have identified two genes that are highly likely to be involved in the development of DAP-R: i) a gene (cls) encoding a cardiolipin synthase enzyme in both VREfs and VREfm, involved in cell membrane homeostasis and ii) a VREfs homolog of the liaF gene, which is part of a three-component gene system involved in the bacterial cell envelope response to antimicrobials and antimicrobial peptides. Thus, we aim to a) investigate the contribution of mutations in the above genes (cls in both VREfs and VREfm and liaF in VREfs) to DAP-resistance, and b) evaluate strategies to optimize the use of DAP for VRE by testing the effect of escalating doses of DAP and combination therapies of DAP with i) ampicillin (for VREfs), and ii) with tigecycline or rifampin (for VREfm), in preventing emergence of DAP-R using a murine model of infective endocarditis. We anticipate that these studies will contribute to a deeper understanding of the role of CM phospholipid homeostasis and cell envelope regulation in the development of antibiotic resistance and antimicrobial peptide action and will certainly facilitate the preservation of DAP as a useful antibiotic to treat VRE infections in the future. PUBLIC HEALTH RELEVANCE: This proposal seeks to understand the molecular strategies used by vancomycin-resistant enterococci (VRE, a common hospital-associated pathogen) to develop resistance to the antibiotic daptomycin;a compound of last resource to treat VRE infections. We aim to design strategies to prevent the emergence of daptomycin resistance in VRE and potentially characterize novel targets for the development of antimicrobial agents with activity against enterococci and other multidrug resistant organisms.